The air flow produced by a pump motor on a personal air sampler is adjusted automatically to maintain a constant air flow without measuring the actual speed of the pump, and without measuring any pressures, by adjusting the speed of the pump motor according to a calibrated relation of speed to power drawn under loading.
Personal air samplers are worn or carried by workers in industrial surroundings and elsewhere to collect particulates, aerosols, microorganisms, chemicals, and other components of the air they breathe. To determine the concentrations of the components in the air, it is necessary to maintain a known flow rate of air over a period of time, preferably a constant flow rate throughout a known time exposure. Factors affecting the ability to provide a constant flow of air in the sampler include the type of pump used, the power supply to the pump, and pressure drops in the air intake, the sampler itself, and the exhaust as the sampler is used. The sampler may have a filter-like particle collector which is expected to cause a resistance to flow as material accumulates on it. A constant flow rate should be maintained in spite of changes in conditions. The present invention is directed to maintaining a constant flow of air through the sampler in spite of resistance to flow caused by random obstructions in the air flow path.
An air sampler as worn or carried by a worker in a factory or other facility typically comprises a small air pump having an air intake, a collector which receives the air from the pump, having an impact surface or a container for collecting aerosols, microorganisms, particulates, and/or chemicals from the air, an exhaust port for the used air, a power supply for the pump, and an electric control for operating the pump. The quantity of collected materials may readily be related to the total volume of air sampled where the air flow is constant and the user accurately records the duration of use.
In the past, complicated circuitry and rechargeable batteries have tended, to make the overall expense of personal samplers inaccessible for some users. A brief review of some of the prior work in the art follows.
Sipin, in U.S. Pat. No. 5,520,517, senses pressure at the outlet of his pump and changes the speed of the pump as dictated by a calibrated relationship of speed to a flow rate over a range of values. Baker et al in U.S. Pat. No. 4,527,953, also monitor actual pressure and in fact maintain a pressure switch operable by pressure drop. Padden, in U.S. Pat. No. 4,747,315, describes a system for adjusting the voltage to, the motor to maintain a desired flow rate.
Peck et al, U.S. Pat. No. 5,101,713, store values representing a relationship between air flow, pump revolutions per minute (RPM), and pulse width modulation (PWM), and then periodically adjust the PWM value to maintain the RPM at a value predetermined to maintain a desired flow rate. See also Arvidson et al U.S. Pat. No.5,313,548.
In U.S. Pat. No. 4,292,574, Sipin et al maintain a constant motor speed in a system which employs a measurement of back EMF from the motor.
The present invention provides a constant flow of air through a personal air sampler regardless of changes in the air flow path, by altering the pump speed as a predetermined function of the power drawn under loading.
A typical personal air sampler to which this invention applies has a collector, an air pump, an air pump motor, a portable power supply, and an electrical circuit. As each sampler may vary slightly from unit to unit, or even if not, the sampler is made to include within its electrical circuit a continuous or intermittent measurement of the voltage across (used by) the pump motor. Also, preferably, the circuitry includes a duty cycle control for application of a pulsed voltage. The voltage measurement includes the detection and measurement of back emf generated by the motor between pulses. The change in back emf is directly proportional to the retardation of the pump motor speed due to a back pressure in the air pathxe2x80x94accordingly I may use the back emf measurement to increase the voltage to the pump motor in order to increase pump speed to a point known to be capable of providing the desired air flow rate. A correction constant c is therefore built into the microprocessor to permit the circuit to act on the detected back emf.